It has been well established that some degree of resistance is elicited in certain parasitic infections by the metabolic antigens of the parasite. Consequently, identification of the functional antigens, and characterization of the host response to these antigens is a necessary prerequisite for any logical attempt to develop a successful vaccine against a filarial parasite. The purpose of the proposed investigation is (1) To conduct integrated ultrastructural and immunological studies on the development and antigenicity of the esophagus and excretory system, the two possible sources of metabolic antigens of the tissue-dwelling stages of B. malayi, a filarid parasite, (2) To perform complementary studies on the host, a non-human primate, and characterize the immunoglobulin response to these metabolic antigens throughout the course of infection and re-infection, in terms of classes and levels of circulating immunoglobulins and antibody titers, determined, respectively, by radial immunodiffusion and fluorescent antibody technique, (3) To identify, by autoradiographic techniques, the route by which the microfilariae absorb nutrients from plasma. The results of these investigations will indicate whether (1) The metabolic antigens of these two organ systems are sufficiently antigenic to make them suitable as potential vaccinating material, (2) What type of immunoglobulin responses are mustered by the host against these antigens, and (3) Indicate the sites of metabolic activity (absorption) in the microfilaria against which specific chemotherapeutic agents could be designed.